AND8391/D Thermal Considerations for the ON Semiconductor Family of Discrete Constant Current Regulators (CCR) for Driving LEDs in Automotive Applications http://onsemi.com APPLICATION NOTE Prepared by: Mike Sweador (AE), David Helzer (PE) ON Semiconductor Introduction Reference to Datasheet The ON Semiconductor Constant Current Regulator (CCR) family of devices offer outstanding regulation for LEDs and other current based loads, such as battery charging circuits. The CCR reduces the complexity of resistor biased designs for sensitive loads, such as LED strings connected in series. The CCR can also be connected in parallel for higher load current applications. The two−terminal CCR requires no external components to regulate at the specified current. These devices can be used wherever a constant current is needed to maintain luminosity under varying voltage conditions. See application note AND8349/D for basic circuit considerations. The purpose of this paper is to explore the temperature and power boundaries for devices in the SOD−123 and SOT−223 packages operating from typical currents of 20 mA to 30 mA in automotive applications. The SOD−123 devices available are rated at 20 mA, 25 mA, and 30 mA. The SOT−223 devices are rated at 25 mA and 30 mA. See Appendix A for device list. The datasheet describes the devices and defines the following terms that will be used throughout this note: Vak = Voltage applied between the Anode and Cathode of the device. Voverhead = VIN − VLEDs Ireg(SS) = The current through the device supplied to the LEDs under steady−state operating conditions (device on w10 sec) Ireg(P) = The current through the device supplied to the LEDs under pulse test conditions (v 300 msec). VR = Reverse Voltage PD = Device power dissipation, typically in mW. TA = Ambient Temperature in °C TJ = Device Junction Temperature in °C The SOD−123 and SOT−223 Datasheet Thermal Characteristics table lists the thermal performance of each device as related to the heat spreader area and thickness. These datasheet tables and curves show thermal specifications and limits with the device junction temperature (TJ) operating at 150°C, the maximum allowable continuous junction temperature. Operating at TJ max continuously is not recommended for long term reliability. Figure 1 shows power dissipation over changes in ambient temperature for the SOD−123 package. Figure 2 shows qJA (°C/W) and PD (W) for various Cu areas and thicknesses. These tables and graphs illustrate the effect of Cu area, thickness and ambient temperature (TA) over the range of −40°C to 85°C, which encompasses the area of interest for automotive LED operation. LED data sheets show an extreme reduction in luminosity above 85°C TA. © Semiconductor Components Industries, LLC, 2009 August, 2009 − Rev. 2 1 Publication Order Number: AND8391/D AND8391/D 700 PD max @ 855C 500 mm2 2 oz 500 mm2 1 oz 600 300 mm2 2 oz 500 300 mm2 1 oz 400 300 100 mm2 2 oz 200 100 −40 500 mm2 2 oz Cu 241 mW 500 mm2 1 oz Cu 228 mW 300 mm2 2 oz Cu 189 mW 300 mm2 1 oz Cu 182 mW 100 mm2 2 oz Cu 117 mW 100 mm2 1 oz Cu 108 mW 100 mm2 1 oz −20 0 20 40 60 80 TA, AMBIENT TEMPERATURE (°C) Figure 1. Power Dissipation vs. Ambient Temperature (SOD−123) @ TJ = 1505C for Variable Copper Heat Spreader 1200 0.6 TA = 25°C 0.5 1000 Power Curve 2.0 oz Cu qJA, (°C/W) 800 0.4 Power Curve 1.0 oz Cu 600 0.3 400 qJA 1.0 oz Cu 0.2 200 qJA 2.0 oz Cu 0.1 0 0 100 200 300 400 500 600 0 700 PCB COPPER AREA (mm2) Figure 2. SOD−123 NSI14030T1G qJA and PD vs. Cu Area http://onsemi.com 2 MAXIMUM POWER (W) PD, POWER DISSIPATION (mW) 800 AND8391/D NOTE: 300 mm2 2 oz Cu area has better thermal performance than 500 mm2 1 oz Cu for this package. Figure 3 shows power dissipation over changes in ambient temperature for the SOT−223 package. Figure 4 shows qJA (°C/W) and PD (W) for various Cu areas and thicknesses. These tables and graphs illustrate the effect of Cu area, thickness and ambient temperature (TA ) over the range of −40°C to 85°C which encompasses the area of interest for automotive LED operation. 2200 PD, POWER DISSIPATION (mW) PD max @ 855C 500 mm2 2 oz 2000 500 mm2 2 oz Cu 300 mm2 2 oz 1800 500 mm2 1 oz 1600 300 mm2 1 oz 1400 1200 100 mm2 1 oz 1000 722 mW 300 mm2 2 oz Cu 676 mW 500 mm2 1 oz Cu 631 mW 300 mm2 1 oz Cu 598 mW 100 mm2 2 oz Cu 559 mW 100 mm2 1 oz Cu 494 mW 800 600 100 mm2 2 oz 400 −40 −20 0 20 40 60 TA, AMBIENT TEMPERATURE (°C) 80 Figure 3. Power Dissipation vs. Ambient Temperature (SOT−223) @ TJ = 1505C 180 1.5 Power Curve 2.0 oz Cu 160 qJA 1.0 oz Cu 140 Power Curve 1.0 oz Cu qJA, (°C/W) 120 1.3 1.2 100 1.1 80 qJA 2.0 oz Cu 60 1 0.9 40 20 0 1.4 TA = 25°C 0 100 200 300 400 500 600 COPPER HEAT SPREADER AREA (mm2) 0.8 0.7 700 Figure 4. SOT−223 qJA and PD vs. Cu Area PC board design and the use of multilayer board material will affect the thermal performance. See ON Semiconductor application notes AND8220/D and AND8222/D for further information. Ambient operating temperature (TA) and estimated device power will help determine which package to use. Figures 2 and 4 can be used to quickly determine which package and heat sink is a good candidate for the application. incoming inspection of a CCR where the test times are a minimum (t v 300 ms). DC steady−state (Ireg(SS)) testing is applicable for application verification where the CCR will be operational for seconds, minutes or hours. ON Semiconductor has correlated the difference in Ireg(P) to Ireg(SS) for stated board material, size, copper area and copper thickness. Ireg(P) will always be greater than Ireg(SS) due to the die temperature rising during Ireg(SS). This heating effect can be minimized during circuit design with the correct selection of board material, metal trace size and weight for the operating current, voltage, and board operating temperature (TA) and package. (Refer to the Thermal Characteristics table in datasheet). Current Regulation: Pulse Mode vs. Steady−State NOTE: All curves are based upon a typical 30 mA CCR device. There are two methods of measuring current regulation: Pulse mode (Ireg(P)) testing is applicable for factory and http://onsemi.com 3 AND8391/D The curves of Figure 5 for the SOD−123 and Figure 6 for the SOT−223 packages show the relationship between Ireg and time. Ireg decreases with time due to the effect of power on the die. Ireg vs. TIME 32 37 TA = 25°C Vak = 7.5 V 36 TA = 25°C Vak = 7.5 V 31.5 31 34 Ireg, (mA) Ireg, (mA) 35 33 32 30.5 30 31 29.5 30 29 0 5 10 15 20 25 30 29 35 0 TIME (s) 5 10 15 20 25 30 35 TIME (s) Figure 5. Typical SOD−123 30 mA, 300 mm2, 1 oz Cu, In Still Air Figure 6. Typical SOT−223 30 mA, 300 mm2, 2 oz Cu, In Still Air Correlation studies show that for each package steady state Ireg there is a corresponding Pulsed Ireg value. Notice on these two−terminal devices that the SOT−223 Ireg(P) has a lower value than the SOD−123 Ireg(P), which results in Ireg(SS) of 30 mA. This is due to the better RqJA of the SOT−223. See Figures 7 and 8. The slope of the line in Figures 7 and 8 will change if the actual footprint and board thermal properties differ from the footprint listed in the figures. STEADY STATE CURRENT (Ireg(SS)) vs. Vak @ 30 mA 35 34 34 33 32 Ireg(SS), (mA) Ireg(SS), (mA) 33 35 TA = 25°C Vak = 7.5 V 31 30 29 TA = 25°C Vak = 7.5 V 32 31 30 29 28 28 27 27 26 26 25 30 31 32 33 34 35 36 37 38 39 40 41 42 43 25 26 27 28 29 30 31 32 33 34 35 36 Ireg(P) (mA) Ireg(P) (mA) Figure 7. Ireg(SS) vs. Ireg(P) Testing SOD−123, 300 mm2, 1 oz Cu, In Still Air Figure 8. Ireg(SS) vs. Ireg(P) Testing SOT−223, 300 mm2, 2 oz Cu, In Still Air http://onsemi.com 4 37 AND8391/D 37 36 35 TA = −40°C 34 33 32 31 30 29 TA = 25°C 28 27 26 25 TA = 85°C 24 3 3.5 4 4.5 5 See ON Semiconductor application note AND8223/D for additional information. SOD−123 devices exhibit a greater negative temperature coefficient as shown in Figure 9 than corresponding SOT−223 devices as shown in Figure 10, due to the difference in the package RqJA. The SOD−123 package reaches thermal saturation with less power applied than the SOT−223 package. [−0.073 mA/°C Typ @ Vak = 7.5 V [−0.059 mA/°C Typ @ Vak = 7.5 V 5.5 6 6.5 7 Vak (V) 7.5 8 8.5 9 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 Ireg(SS), (mA) Ireg(SS), (mA) The negative temperature coefficient trend of a SOD−123 CCR has a benefit as it avoids thermal runaway. There are two areas of interest on the curves of Figure 9. The first is for a given TA. Each curve shows a decrease in Ireg(SS) as Vak increases and therefore PD increases. There also is the ambient temperature affect on Ireg for a fixed Vak condition. Both the SOD−123 (Figure 9) and SOT−223 (Figure 10) show a decrease in Ireg(SS) as TA increases. 9.5 10 TA = 85°C TA = 125°C 3 4.5 5 5.5 6 6.5 7 Vak (V) 7.5 8 8.5 9 9.5 10 Example 2: Three Red LEDs with each having a VF of 2.0 Vdc @ 30 mA. Automotive battery voltage of 16 Vdc. Ambient temperature max of 85°C. Available heat sink area for device is 300 mm2 of 1 oz Cu. PD of device = (16 Vdc – (3 x 2.0 Vdc) + 0.2 Vdc) x 30 mA = 294 mW SOD−123 PD max @ 85°C, 300 mm2 of 1 oz Cu = 182 mW SOT−223 PD max @ 85°C, 300 mm2 of 1 oz Cu = 598 mW The SOT−223 gives a margin of safety in the application. Or, knowing that 294 mW of power needs to be dissipated, we can select a SOT−223 device using 100 mm2 of 1 oz Cu. For a series circuit (Figure 11), the power dissipation of the CCR is determined by: (Vsource – (VLEDS + VRPD)) x Ireg. Using the worst case scenario; i.e, highest Vsource, Lowest LED VF, and highest target Ireg. Using a 16 V source (auto voltage regulator high output) driving two white LEDs with a Vf of 4.2 V, a reverse protection diode (RPD) with a VF of 0.2 V and 30 mA Ireg would give: (16 V − (2 x 4.2 V + 0.2 V)) x 0.030 A = 7.4 V x 0.03 A = 222 mW. For an ambient temperature of 85°C, from the PD curves of Figures 1 and 3 a SOD−123 with 500 mm2 1 oz Cu would 1 3.5 4 suffice. A SOT−223 with 100 mm2 1 oz Cu would also work. Circuit Design Example 1: −DC [−0.061 mA/°C Typ @ Vak = 7.5 V Figure 10. Typical SOT−223 30 mA, 300 mm2, 2 oz Cu, In Still Air The following design examples will show how to determine which package device and the Cu needed for a simple circuit. 1 [−0.058 mA/°C Typ @ Vak = 7.5 V TA = 25°C Figure 9. Typical SOD−123 30 mA, 300 mm2, 1 oz Cu, In Still Air +DC [−0.088 mA/°C Typ @ Vak = 7.5 V TA = −40°C Reverse Battery Protection Diode (RPD) D1 Anode MBRS140T3 Q1 CCR NSI45030T1G Cathode Automotive LED’s (3 mm2 − 4 Lead) D3 D4 D2 1 2 1 2 2 LED LED Figure 11. http://onsemi.com 5 LED AND8391/D Ireg(SS), (mA) The following graphs show the relationship between Ireg(SS) and TA for both the SOD−123 and SOT−223 for a stated Cu area and thickness in still air. They also give the slope of the line which can be used to estimate TJ at a specific TA. 36 PD [ 260 mW 35 34 33 32 Est. T [ 54°C J 31 Between −40°C & 25°C 30 −0.073 mA/°C 29 28 27 26 25 24 23 −40 −30 −20 −10 0 10 The formula for estimating TJ is: TJ = (PD x RqJA) + TA (RqJA value from datasheet) For the SOD−123 @ 25°C, TJ = (225 mW x 360°C/W) + 25°C = 106°C (as shown on the graph). Ireg(SS) vs. TA RqJA [ 360°C/W Vak = 7.5 V PD [ 225 mW PD [ 198 mW Est. TJ [ 106°C Between 25°C & 85°C −0.059 mA/°C Est. TJ [ 156°C 20 30 40 50 60 70 80 TA (°C) Ireg(SS), (mA) Figure 12. Typical SOD−123 30 mA, 300 mm2, 1 oz Cu, In Still Air 36 PD [ 268 mW 35 34 33 Est. TJ [ −14°C 32 Between −40°C & 25°C 31 −0.088 mA/°C 30 29 28 27 26 25 24 23 −40 0 −30 −20 −10 RqJA [ 96°C/W Vak = 7.5 V PD [ 225 mW Est. TJ [ 47°C PD [ 192 mW Between 25°C & 85°C −0.072 mA/°C Est. TJ [ 103°C 10 20 30 40 TA (°C) 60 70 80 Between 85°C & 125°C −0.061 mA/°C PD [ 174 mW 90 Est. TJ [ 142°C 100 110 Figure 13. Typical SOT−223 30 mA, 300 mm2, 2 oz Cu, In Still Air PWM Current Control CCR Anode The power dissipation of the CCR can be reduced when used in a pulse width modulation (pwm) controlled circuit Figure 14. The dc average current will be Ireg(SS) x duty cycle %. For a typical 30 mA CCR at 20% duty cycle, TA of 25°C, the average current through the LEDs will be 6.0 mA. Lead Input CCR Cathode Control Input Output Figure 14. http://onsemi.com 6 120 130 AND8391/D R(t) for 300 mm2 of 1 oz Cu for a SOD−123 from Figure 15 would be [ 90°C/W. Therefore; 216 mW x 90°C/W = 19.4°C temperature rise. The device and heat sink will require analysis for worst case condition to account for 100% duty cycle. Figures 15 and 16 will assist to determine the temperature rise caused by a power pulse. Example: If the control input is a 500 Hz, 20% duty cycle pwm applied to the three red LED circuit of Figure 11, the 1000 R(t) (°C/W) 50% Duty Cycle 100 20% 10 10% 5% 2% 1% 1 Single Pulse 0.1 0.000001 0.00001 0.0001 0.001 0.01 0.1 1 10 100 1000 100 1000 PULSE TIME (s) Figure 15. SOD−123 NSI45030T1G PCB Cu Area 300 mm2 PCB Cu thk 1.0 oz 1000 5% R(t) (°C/W) 100 50% Duty Cycle 20% 10% 10 2% 1 1% Single Pulse 0.1 0.000001 0.00001 0.0001 0.001 0.01 0.1 1 10 PULSE TIME (s) Figure 16. CCR SOT−223 NSI45030ZT1G PCB Cu Area 300 mm2 PCB Cu thk 2.0 oz Summary: The thermal behavior of a CCR is generalized in the following matrix: TA ↑ Heatsink Area ↑ Vak ↑ Ireg(SS) ↓ ↑ NC* TJ ↑ ↓ ↑ *In general SOD−123 for 3 V < Vak < 10 V, all other variables constant: Ireg(SS) changes < 2 mA (less @ TA > 25°C). In general SOT−223 for 3 V < Vak < 10 V, all other variables constant: Ireg(SS) changes < 3 mA. Figure 17. http://onsemi.com 7 AND8391/D APPENDIX A SOD−123 devices are: SOT−223 devices are: NSI45020T1G, Steady State Ireg(SS) = 20 mA $15% NSI45025T1G, Steady State Ireg(SS) = 25 mA $15% NSI45030T1G, Steady State Ireg(SS) = 30 mA $15% NSI45020AT1G, Steady State Ireg(SS) = 20 mA $10% NSI45025AT1G, Steady State Ireg(SS) = 25 mA $10% NSI45030AT1G, Steady State Ireg(SS) = 30 mA $10% NSI45025ZT1G, Steady State Ireg(SS) = 25 mA $15% NSI45030ZT1G, Steady State Ireg(SS) = 30 mA $15% NSI45025AZT1G, Steady State Ireg(SS) = 25 mA $10% NSI45030AZT1G, Steady State Ireg(SS) = 30 mA $10% APPENDIX B Application Note Title AND8349/D Automotive Applications The Use of Discrete Constant Current Regulators (CCR) For CHMSL Lighting AND8220/D How To Use Thermal Data Found in Data Sheets AND8222/D Predicting the Effect of Circuit Boards on Semiconductor Package Thermal Performance AND8223/D Predicting Thermal Runaway The products described herein (NSI45020T1G, NSI45025T1G, NSI45030T1G, NSI45020AT1G, NS145025A51G, NSI45030AT1G, NSI45025ZT1G, NSI45030ZT1G, NSI45025AZT1G, NSI45030AZT1G) have patents pending. 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